Controlling web tension by actively controlling velocity of dancer roll

ABSTRACT

This invention pertains to processing continuous webs such as paper, film, composites, and the like, in dynamic continuous processing operations. More particularly, it relates to controlling tension in such continuous webs during the processing operation. Tension is controlled in a dancer control system by connecting a corresponding dancer roll to a servo motor or the like, sensing position, tension, and velocity parameters related to the web and the dancer roll, and providing active gain force commands to cause translational movement in the dancer roll to control temporary, short-term tension disturbances in the web. In some applications of the invention, the dancer control system is used to attenuate short-term tension disturbances. In other applications of the invention, the dancer control system is used to create short-term tension disturbances.

This is a divisional application of copending application Ser. No.08/382,110, filed on Jan. 31, 1995 pending.

FIELD OF THE INVENTION

This invention relates to the processing of continuous webs such aspaper, film, composites, or the like, in dynamic continuous processingoperations. More particularly, it relates to controlling tension in suchcontinuous webs during the processing operation.

BACKGROUND OF THE INVENTION

In the paper and plastic film industries, a dancer roll is widely usedas a buffer between first and second sets of driving rolls, or first andsecond nips, which drive a continuous web. The dancer roll, which ispositioned between the two sets of driving rolls, may also be used todetect the difference in speed between the first and second sets ofdriving rolls.

Typically, the basic purpose of a dancer roll is to maintain constantthe tension on the continuous web which traverses the span between thefirst and second sets of driving rolls, including traversing the dancerroll.

As the web traverses the span, passing over the dancer roll, the dancerroll moves up and down in a track, serving two functions related tostabilizing the tension in the web. First, the dancer roll provides adamping effect on intermediate term disturbances in the tension in theweb, e.g. disturbances that last more than 10 seconds. Second, thedancer roll temporarily absorbs the difference in drive speeds betweenthe first and second sets of driving rolls, until such time as the drivespeeds can be appropriately coordinated.

Typically, the dancer roll is suspended on a support system, wherein agenerally static force supplied by the support system supports thedancer roll against an opposing force applied by the tension in the weband the weight of the dancer roll. So long as the tension in the web isconstant, the dancer roll remains generally centered in its operatingwindow on the track.

When the web encounters an intermediate or long term tensiondisturbance, temporarily increasing or decreasing the tension in theweb, the imbalances of forces on the dancer roll cause translationalmovement in the dancer roll to temporarily restore the tension, andthereby the force balance. So when difference in the speeds of the firstand second sets of drive rolls tend to accord a change in the webtension, the dancer roll temporarily maintains the tension.

Thus, the dancer roll generally stabilizes the tension in the web, bycompensating for temporary changes in the operating tension. While thedancer roll, as conventionally used, provides valuable functions, italso has its limitations.

To the extent the tension disturbance is long term, such as a speedimbalance in the sets of driving rolls, the function of the dancer rollis only temporary, to accommodate the disturbance until resolution isaddressed at the source by changing driving speeds. Such changing of thedrive speeds is commonly known.

Conversely, to the extent the tension disturbance is short term, such asno more than 10 seconds, the mass and corresponding inertia of thedancer roll prevent the dancer from providing a meaningful responsebased on the static forces, during the period of existence of thedisturbance. The response time based on the gravitational accelerationprovided by the static forces is simply too slow to overcome the inertiaof the dancer system in time to effectively counter such short-termtension disturbances.

It is known to provide an active drive to the dancer roll in order toreduce the response time in a static system, wherein the web is heldunder tension, but is not moving along the length of the web, wherebythe dynamic disturbances, and the natural resonance frequencies of thedancer roll and the web are not accounted for, and whereby the resultingoscillations of the dancer roll can become unstable. Kuribayashi et al,"An Active Dancer Roller System for Tension Control of Wire and Sheet."University of Osaka Prefecture, Osaka, Japan, 1984.

However, it is not known to provide an active dancer roll in a dynamicsystem wherein dynamic variations in operating parameters are used tocalculate variable active response force components for applying activeand variable forces to the dancer roll, and wherein appropriate gainconstants are used to compute effective response time without allowingthe system to become unstable.

It is an object of the invention to provide methods and apparatus forcontrolling tension in a moving web, using a dancer roll, applying tothe dancer roll a force having an active component, and adjusting thevalue of the active component at least one time per second.

It is another object to provide such methods and apparatus forcontrolling tension in a moving web, including sensing tension withsufficient frequency to identify a tension disturbance which exists for10 seconds or less, and adjusting the active component of the force onthe dancer roll at least five times during the existence of the tensiondisturbance.

Another object is to provide such methods and apparatus, includingadjusting the value and direction of the active force componentaccording to the equation

    T*.sub.dancer =r[F.sub.d static +b.sub.a (V*.sub.p -V.sub.p)+k.sub.a (F*.sub.c -F.sub.c)].

A related object is to provide methods and apparatus for controlling thetranslational movement of the dancer roll such that the dancer roll,itself, creates controlled temporary and desirable tension disturbances.

Another objective is to provide methods and apparatus for providing suchcontrolled tension disturbances repetitiously.

SUMMARY OF THE INVENTION

This invention describes apparatus and methods for controlling tensionand tension disturbances in a continuous web during processing of theweb. In a first aspect, the invention can be used to attenuate undesiredtension disturbances in the web. In a second aspect, the invention canbe used to create desired tension disturbances in the web.

In a typical converting process, a parent roll of paper, composite, orlike web of raw material is unwound at one end of a processing line, andis processed through the processing line to thereby convert the rawmaterial, such as to shorter or narrower rolls of product, or to shapeproducts from the raw material, to separate products from the rawmaterial, and/or to combine the raw material with other input elementsto thereby create a product or product precursor. Such processingoperations are generally considered "continuous" processes because theroll of raw material generally runs "continuously" for an extendedperiod of time, feeding raw material to the processing system.

In such continuous processes, it is common to employ one or more dancerrolls to sense and control intermediate and longer term changes in thetension of the web. The focus of this invention is to sense and controlshort-lived disturbances in the tension of the web, and to sense the webtension with sufficient frequency that disturbances are detected earlyin their lives. Counteracting variable forces are applied to the dancerroll during the short lives of the disturbances, effecting activetranslational movement of the dancer roll that attenuates the effect ofthe short term tension disturbances.

A first family of embodiments of the invention is illustrated in amethod of controlling web tension in a processing operation wherein acontinuous web of material is advanced through a processing step andwherein the web experiences an average dynamic tension along a givensection of the web. The method of controlling the tension in therespective section of web comprises the steps of providing a dancer rolloperative on the respective section of web; applying a first generallystatic force component to the dancer roll, through the first generallystatic force component having a first value and direction, generallybalancing the dancer roll against the average dynamic tension in therespective section of web, the dancer roll being passively responsive,through the first generally static force component, to therebycompensate for changes in web tension lasting longer than about 10seconds; and applying a second active and variable force component tothe dancer roll, the second variable force component having a secondvalue and direction, modifying the first generally static forcecomponent, and thereby modifying (i) the effect of the first generallystatic force component on the dancer roll and (ii) the correspondingtranslational velocity of the dancer roll. The overall result is thatthe net translational velocity of the dancer roll is controlled by theadditive result of the first generally static force component and thesecond active force component.

The method preferably includes adjusting the value and direction of thesecond variable force component, each such adjusted value and directionof the second variable force component (i) replacing the previous suchvalue and direction of the second variable force component and (ii)acting in combination with the first static force component to provide anet translational velocity to the dancer roll. The preferred frequencyof adjustment depends on the frequency of tension disturbances to beimposed or attenuated. Typical frequency is at least 1 time per secondand, depending on the application, may be at least 500 times per second,or up to about 1000 times per second or more.

In preferred embodiments of the first family, the method includessensing tension in the respective section of the web at least 1 time persecond, preferably at least 500 times per second, more preferably up toabout 1000 times per second, or more, recomputing the value anddirection of the second variable force component using the sensedtension, and thereby adjusting the value and direction of the computedsecond variable force component at least 1 time per second, preferablyat least 500 times per second, more preferably up to about 1000 timesper second or more, and applying the recomputed value and direction tothe dancer roll at a corresponding frequency.

Another way to approach the sensing step is sensing tension in therespective section of the web with sufficient frequency to identify atension disturbance, in the web, which exists for a period of no morethan about 10 seconds, preferably no more than about 0.67 second, morepreferably no more than about 0.33 second, and most preferably no morethan about 0.2 second, and using sensed tension to compute and therebyadjust the value and direction of the second variable force component atan adjustment frequency providing at least about five adjustments,preferably at least 100 adjustments, and more preferably at least 200adjustments during the existence of each tension disturbance.

A second family of embodiments is illustrated in a method of controllingweb tension in a similar processing operation wherein a continuous webof material is advanced through a processing step wherein the webexperiences an average dynamic tension along a given section of the web.In this family of embodiments, the method of controlling the tension inthe respective section of web comprises the steps of providing a dancerroll operative on the respective section of web; providing a servo motorconnected to the dancer roll and thereby providing an actuating force tothe dancer roll; measuring a first velocity of the web after the dancerroll; measuring a second velocity of the web at the dancer roll;measuring translational velocity of the dancer roll; sensing theposition of the dancer roll; measuring web tension before and after thedancer roll; and controlling the servo motor with a computer controllerwhich provides control commands to the servo motor based on the sensedposition and the measured tensions and velocities, and therebycontrolling the actuating force imparted to the dancer roll by the servomotor.

The method preferably includes computing a control force command withthe computer controller and thereby computing the torque output commandfor the servo motor, using the equation:

    T*.sub.dancer =r[F.sub.d static +b.sub.a (V*.sub.p -V.sub.p)+k.sub.a (F.sub.c)]

wherein the dancer translational velocity set-point V_(p) * iscalculated using the equation:

    V*.sub.p =EA.sub.o /(EA.sub.o -F.sub.o)[V.sub.2 (1-F.sub.b /EA.sub.o)-V.sub.3 (1-F.sub.o /EA.sub.o)]

and controlling the servo motor based on the torque output command socalculated.

As in the first family of embodiments, the computer controller typicallyprovides control commands to the servo motor at a frequency of at leastone time per second, preferably at least 500 times per second, mostpreferably up to at least about 1000 times per second, or more.

A third family of embodiments is also illustrated in a method ofcontrolling web tension in a similar processing operation wherein acontinuous web of material is advanced through a processing step whereinthe web experiences an average dynamic tension along a given section ofthe web. In this third family of embodiments, the method of controllingthe tension in the respective section of web comprises providing adancer roll operative on the respective section of web; applying a firstgenerally static force component, having a first value and direction, tothe dancer roll, generally balancing the dancer roll against the averagedynamic tension in the respective section of web, the dancer roll beingpassively responsive, through the first generally static forcecomponent, to thereby compensate for changes in web tension which lastlonger than about 10 seconds; sensing the position of the dancer rollwithin an operating window; controlling the position of the dancer rollby changing the relative speeds at which the web is (i) fed to thedancer roll and (ii) taken away from the dancer roll; and sensing ashort duration tension disturbance in the web, existing for a period ofno more than about 10 seconds, preferably no more than about 0.67second, more preferably no more than about 0.33 second, most preferablyno more than about 0.2 second, and imparting a correspondingcounteracting translational force component to the dancer roll, andthereby attenuating the effect of the short duration tensiondisturbance.

While the instant above discussions relate to attenuating tensiondisturbances, the same general system (methods and apparatus) can beadapted to create a tension disturbance existing for generally no morethan about 10 seconds in the web, preferably no more than about 0.67second, more preferably no more than about 0.33 second, most preferablyno more than about 0.2 second, by applying to the dancer roll atemporary and unbalancing force of corresponding short duration, therebycausing a temporary translational movement of the dancer roll andcorresponding disturbance of the tension in the web.

A fourth family of embodiments of the invention comprehends processingapparatus for advancing a continuous web of material through aprocessing step wherein the web experiences an average dynamic tensionalong a given section of the web. The processing apparatus comprises adancer roll operative for controlling tension on the respective sectionof web; actuating means for applying a first generally static forcecomponent to the dancer roll, through the first generally static forcecomponent having a first value and direction, and generally balancingthe dancer roll against the average dynamic tension in the respectivesection of the web, the dancer roll being passively responsive, throughthe first generally static force component, to thereby compensate forchanges in web tension which last longer than about 10 seconds, and forapplying a second active and variable force component to the dancerroll, the second active and variable force component having a secondvalue and direction, modifying the first generally static forcecomponent such that net translational velocity of the dancer roll iscontrolled by the net actuating force.

Preferably, the processing apparatus includes a computer controller,connected to the actuating means, for controlling the actuating forceimparted to the dancer roll by the actuating means, and for recomputingand thereby adjusting the value and direction of the second variableforce component at least 1 time per second, more preferably at least 500times per second, still more preferably up to about 1000 times persecond or more, each such value and direction of the second variableforce component (i) replacing the previous such value and direction and(ii) acting in combination with the first static force component toprovide a net translational velocity to the dancer roll.

The processing apparatus preferably includes sensing means for sensingtension in the respective section of the web, the sensing means beingadapted for sensing tension in the respective section of the web atleast 1 time per second, more preferably at least 500 times per second,still more preferably up to about 1000 times per second or more, thecomputer controller being connected to the actuating means, and beingadapted for recomputing the value and direction of the second variableforce component and thereby adjusting the value and direction of thecomputed second variable force component at least 1 time per second,preferably at least 500 times per second, more preferably up to about1000 times per second or more, the actuating means being adapted toapply the recomputed second variable force component to the dancer roll,a corresponding number of times, according to the values and directionscomputed by the computer controller.

The sensing means preferably has frequency sensitivity sufficient toidentify a tension disturbance in the web which exists for a period ofno more than about 10 seconds, preferably no more than about 0.67second, more preferably no more than about 0.33 second, most preferablyno more than about 0.2 second, with the computer controller beingadapted to recognize the period of existence of recurring such tensiondisturbances and to provide a response operative to adjust the value anddirection of the second variable force component at an adjustmentfrequency providing an average of at least about five adjustments,preferably at least 100 adjustments, and more preferably at least 200adjustments during the existence of each such tension disturbance.

A fifth family of embodiments of the invention comprehends processingapparatus for advancing a continuous web of material through aprocessing step wherein the web experiences an average dynamic tensionalong a given section of the web. The processing apparatus comprises adancer roll operative for controlling tension on the respective sectionof web; a servo motor connected to the dancer roll and thereby providingan actuating force to the dancer roll; first means for measuring a firstvelocity of the web at the dancer roll; second means for measuring asecond velocity of the web after the dancer roll; third means formeasuring translational velocity of the dancer roll; fourth means forsensing the position of the dancer roll; fifth means for measuring webtension before the dancer roll; sixth means for measuring web tensionafter the dancer roll; and a computer controller for providing controlcommands to the servo motor based on the sensed position of the dancerroll, and the measured tensions and velocities, and thereby controllingthe actuating force imparted to the dancer roll by the servo motor.

The computer controller is preferably adapted to compute a control gainforce command, and thereby computing the torque output command for theservo motor, using the formula

    T*.sub.dancer =r[F.sub.d static +b.sub.a (V*.sub.p -V.sub.p)+k.sub.a (F*.sub.c -F.sub.c)]

wherein the dancer translational velocity set-point V*_(p) is calculatedusing the formula ##EQU1## and to control the servo motor based on thetorque output command so calculated, the computer controller typicallybeing adapted to provide control commands to the servo motor at afrequency of at least 1 time per second, preferably at least 500 timesper second, more preferably at least 1000 times per second.

In a sixth family of embodiments, the invention provides processingapparatus for advancing a continuous web of material through aprocessing step wherein the web experiences an average dynamic tensionalong a given section of the web. The processing apparatus comprises adancer roll operative on the respective section of web; means forapplying a first generally static force component, having a first valueand direction, to the dancer roll, generally balancing the dancer rollagainst the average dynamic tension in the respective section of web,the dancer roll being passively responsive, through the first generallystatic force component, to thereby compensate for changes in web tensionlasting longer than about 10 seconds; a first sensing and control systemfor sensing the position of the dancer roll within an operating window,and controlling the position of the dancer roll by changing the relativespeeds at which the web is (i) fed to the dancer roll and (ii) takenaway from the dancer roll; and a second sensing and control system forsensing, in the web, a short duration tension disturbance existing for aperiod of no more than about 10 seconds, and for imparting acorresponding counteracting force to the dancer roll, and therebyattenuating the effect of the tension disturbance.

While the instant above discussions relate to attenuating tensiondisturbances, generally, the same sensing and control equipment can beadapted to create a tension disturbance existing for generally no morethan about 10 seconds in the web by applying, to the dancer roll, atemporary and unbalancing force of corresponding short duration, therebycausing a temporary translational movement of the dancer roll andcorresponding temporary disturbance of the tension in the web.Preferably, the second sensing and control system can create a tensiondisturbance having even shorter duration, such as no more than about0.67 second, preferably no more than about 0.33 second, most preferablyno more than about 0.2 second.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood and furtheradvantages will become apparent when reference is made to the followingdetailed description of the invention and the drawings, in which:

FIG. 1 is a pictorial view of part of a conventional processingoperation, showing a dancer roll adjacent the unwind station.

FIG. 2 is a pictorial view of one embodiment of the invention, againshowing a dancer roll adjacent the unwind station.

FIG. 3 is a flow diagram representing a control system of the invention.

FIG. 4 is a free body force diagram showing the forces acting on thedancer roll.

FIG. 5 is a control system block diagram.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The following detailed description is made in the context of a paperconverting process. The invention can be appropriately applied to otherflexible web processes.

FIG. 1 illustrates a typical conventional dancer roll control system.Speed of advance of web material is controlled by an unwind motor 14 incombination with the speed of the nip downstream of the dancer roll. Thedancer system employs lower turning rolls before and after the dancerroll, itself. The dancer roll moves vertically up and down within theoperating window defined between the lower turning rolls and the upperturning pulleys in the endless cable system. The position of the dancerroll in the operating window, relative to (i) the top of the windowadjacent the upper turning pulleys and (ii) the bottom of the windowadjacent the turning rolls is sensed by the position transducer 2. Agenerally static force having a vertical component is provided to thedancer roll support system by the air cylinder 3.

In general, to the extent the process take-away speed exceeds the speedat which web material is supplied to the dancer roll, the static forceson the dancer roll cause the dancer roll to move downwardly within itsoperating window. As the dancer roll moves downwardly, the change inposition is sensed by the position transducer 2, which sends acorrective signal to the unwind motor 14 to increase the speed of theunwind. The speed of the unwind increases enough to return the dancerroll to the mid-point in its operating window.

By corollary, if the take-away speed lags the speed at which webmaterial is supplied to the dancer roll, the static forces on the dancerroll cause the dancer roll to move upwardly within its operating window.As the dancer roll moves upwardly, the change in position is sensed bythe position transducer 2. As the dancer rises above the mid-point inthe operating window, the position transducer 2 sends a correspondingcorrective signal to the unwind motor 14 to decrease the speed of theunwind, thereby returning the dancer roll to the mid-point in theoperating window.

The above conventional dancer roll system is limited in that itsresponse time is controlled by the gravitational contribution tovertical acceleration of the dancer roll, and by the mass of equipmentin e.g. the unwind apparatus that must change speed in order to effect achange in the unwind speed.

Referring to FIG. 2, the process system 10 of the invention incorporatesan unwind 12, including unwind motor 14 and roll 16 of raw material. Aweb 18 of the raw material is fed from the roll 16, through a dancersystem 20, to the further processing elements of the converting processdownstream of the dancer system 20.

In the dancer system 20, the web of material 18 passes under turningroll 22 before passing over the dancer roll 24, and passes under turningroll 26 after passing over the dancer roll 24. As shown, the dancer roll24 is carried by a first endless drive cable 28.

Starting with a first upper turning pulley 30, the first endless drivecable 28 passes downwardly as segment 28A to a first end 32 of thedancer roll, and is fixedly secured to the dancer roll at the first end32. From the first end 32 of the dancer roll, the drive cable continuesdownwardly as segment 28B to a first lower turning pulley 34, thencehorizontally under the web 18 as segment 28C to a second lower turningpulley 36. From second lower turning pulley 36, the drive cable passesupwardly as segment 28D to a second upper turning pulley 38. From secondupper turning pulley 38, the drive cable extends downwardly as segment28E to the second end 40 of the dancer roll, and is fixedly secured tothe dancer roll at the second end 40. From the second end 40 of thedancer roll, the drive cable continues downwardly as segment 28F to athird lower turning pulley 42, thence back under the web 18 as segment28G to the fourth lower turning pulley 44. From fourth lower turningpulley 44, the drive cable extends upwardly as segment 28H to, and isfixedly secured to, connecting block 46. From connecting block 46, thedrive cable continues upwardly as segment 28I to the first upper turningpulley 30, thus completing the endless loop of drive cable 28.

Connecting block 46 connects the endless drive cable 28 to an endlessdrive chain 48. From the connecting block 46, the endless drive chain 48extends upwardly as segment 48A to a third upper turning pulley 50. Fromupper turning pulley 50, the endless drive chain extends downwardly assegment 48B to fifth lower turning pulley 52. From fifth lower turningpulley 52, the drive chain extends back upwardly as segment 48C to theconnecting block 46, thus completing the endless loop of drive chain 48.

Shaft 54 connects the fifth lower turning pulley 52 to a first end ofservo motor 56. Dancer roll position sensor 58 and dancer rolltranslational velocity sensor 60 extend from the second end of servomotor 56, on shaft 61.

Load sensors 62, 64 are disposed on the ends of turning rolls 22, 26respectively for sensing stress loading on the turning rolls transverseto their axes, the stress loading on the respective turning rolls 22, 26being interpreted as tension on the web.

Velocity sensor 66 is disposed adjacent the end of turning roll 26 tosense the turn speed of turning roll 26. Velocity sensor 68 is disposedadjacent the second end 40 of dancer roll 24 to sense the turn speed ofthe dancer roll, the turning speeds of the respective rolls beinginterpreted as corresponding to web velocities at the respective rolls.

The dancer system 20 is controlled by computer controller 70. Computercontroller 70 is a conventional digital computer outfitted withconventional signal interface, which can be programmed in conventionallanguages such as "Basic," "Pascal," "C," or the like. Such computersare generically known as "personal computers," and are available fromsuch manufacturers as Compaq® and IBM®. Suitable signal interface areavailable from such manufacturers as Keithley Metrabyte® and ComputerBoards, Inc.®

Position sensor 58, velocity sensors 60, 66, 68, and load sensors 62, 64all feed their inputs into the computer controller 70. Computercontroller 70 processes the several inputs, computing a velocity setpoint

    V*.sub.p =EA.sub.o /(EA.sub.o -F.sub.o)[V.sub.2 (1-F.sub.b /EA.sub.o)-V.sub.3 (1-F.sub.o /EA.sub.o)]

and a target servo motor torque command according to

T*_(dancer) =r[F_(d) static +b_(a) (V(_(p) -V_(p))+k_(a) (F*_(c)-F_(c))]

where

F_(d) static =Mg+2F*_(c)

using the following variables:

F_(d) static =Static force component on the dancer roll

F_(c) =Tension in the web after the dancer roll

F*_(c) =Tension in the web, target set point, per process designparameters

F_(b) =Tension in the web ahead of the dancer roll

b_(a) =Control gain constant re dancer translational velocity, in newtonseconds/meter

k_(a) =Control gain constant re web tension

Mg=Mass of the dancer roll times gravity.

V_(p) =Instantaneous translational velocity of the dancer rollimmediately prior to application of the second variable force component

V₂ =Velocity of the web at the dancer roll

V₃ =Velocity of the web after the dancer roll

V*_(p) =Translational velocity of dancer roll, set point

r=Radius of pulley on the servo motor

E=Modulus of elasticity of the web

A_(o) =Cross-sectional area of the unstrained web

T*_(dancer) =Servo motor torque command

V*_(p) represents the target translational velocity of the dancer roll24, to be reached if the set point V*_(p) is not subsequently adjustedor otherwise changed.

The response time is affected by the value selected for the gainconstant "b_(a)." The gain constant "b_(a) " is selected to impose adamping effect on especially the variable force component of theresponse, in order that the active variable component of the responsenot make the dancer roll so active as to become unstable, such as wherethe frequency of application of the responses approaches a naturalresonant frequency of the dancer roll. Accordingly, the gain constant"b_(a) " acts somewhat like a viscous drag in the system. For example,in a system being sampled and controlled at 1000 times per second, andthe mass of the dancer is 1 kg, a maximum control gain constant "b_(a) "is 2000.

Similarly, the gain constant "k_(a) " compensates generally for webtension errors in the system A typical gain constant "k_(a) " for theinstantly above described processing system is 20,000.

It is contemplated that the operation and functions of the inventionhave become fully apparent from the foregoing description of elementsand their relationships with each other, but for completeness ofdisclosure, the usage of the invention will be briefly describedhereinafter.

In a first embodiment of the method of using the invention, a primaryobjective of the dancer system 20 is to attenuate short term tensiondisturbances in the web. Such short term tension disturbances mightcome, for example from unintended, but nonetheless normal, vibrationsemanating from equipment downstream of the dancer roll 24, for examplebearing vibration, motor vibration, and the like. In the alternative,such tension disturbances can also come from tension disturbances whichare intentionally imposed on the web as the web is processed. An exampleof such intentional tension disturbances is shown in U.S. Pat. No.4,227,952 Sabee herein incorporated by reference to show a tensiondisturbance being created with the formation of each tuck or pleat inthe web of material being processed.

Whether the tension disturbances are imposed intentionally orunintentionally, the effect on the web is generally the same. As the web18 traverses the processing system 10, the web is exposed to an averagedynamic tension, representing a normal range of tensions as measuredover a span of the web, for example between roll 16 of raw material andthe next nip 72 downstream of the dancer roll 24, without consideringshort term tension disturbances that last for 10 seconds or less.

In order for the dancer roll to operate as a "dancer" roll, the severalforces acting on the dancer roll must still, in general, be balanced, asshown in FIG. 4. As shown in FIG. 4, the forces applied by the servomotor are balanced against the tension forces in the web, the weight ofthe dancer roll, any existing viscous drag effects times the existingtranslational velocity of the dancer roll, any existing spring effectstimes the change in positioning of the dancer roll, and the dancer masstime its vertical acceleration at any given time.

The servo motor force generally includes a first generally static forcecomponent F_(d) static, having a relatively fixed value, responsive tothe relatively fixed static components of the loading on the dancerroll. The generally static force component F_(d) static provides thegeneral support that keeps the dancer roll more-or-less centeredvertically in its operating window, between the turning rolls 22, 26 andthe upper turning pulleys 30 and 38, responding based on the staticforce plus gravity. To the extent the dancer roll spends significanttime outside a central area of the operating window, the computer 70sends conventional commands to the line shaft drivers or the like toadjust the relative speeds between e.g. the unwind 12 and the nip 72 inthe conventional way to thus bring the dancer roll generally back to thecenter of its operating window.

In addition to the static force component F_(d) static, the servo motor56 exerts a dynamically active, variable force component, responsive toshort-term tension disturbances in the web. The variable forcecomponent, when added to the static force component, comprehends the netvertical force command issued by the computer, to the servo motor. Theservo motor 56 expresses the net vertical force command as torqueT*_(dancer) delivered through the drive chain 48, drive cable 28, andconnecting block 46, to the dancer roll. Accordingly, in addition to thenormal passive response of the dancer roll, based on such static forcesas mass, gravity, and web tension, the dancer control system of theinvention adds a dynamic control component, outputted at the servomotor. The result is a punctuation of the normal dancer system responsecharacteristic with short-term vertical forces being applied to thedancer by the servo motor, with the result that the dancer roll is muchmore pro-active, making compensating changes in translational velocitymuch more frequently than a conventional dancer system that respondsonly passively. Of course, net translational velocity at any given pointin time can be a positive upward movement, a negative downward movement,or no movement at all, corresponding in zero net translational velocity,all depending on the output command from the computer controller. Thecomputer controller 70, of course, computes both the value and directionof the variable force, as well as the net force.

The general flow of information and commands in a command sequence usedin controlling the dancer system 20 is shown in block diagram format inFIG. 3. As seen therein, in step 1 in the command sequence, the variableparameters V_(p), P, F_(b), F_(c), V₂, and V₃ are measured.

In step 2, the variables are combined with the known constants in thecomputer, and the computer computes V*_(p).

In step 3, V*_(p) is combined with additional static values to computethe new force command.

In step 4, the new force command is combined with a servo constant "r"to arrive at the proportional torque command T*_(dancer) outputted fromthe servo motor to the dancer roll through the drive chain 48 and drivecable 28.

In step 5, the sequence is repeated as often as necessary to obtain aresponse that controls the tension disturbances extant in the web underthe dynamic conditions to which the web is exposed.

In general, the tension disturbances of interest in this invention aredisturbances which can be attenuated within about 10 seconds, or less,by appropriate response through the novel combination of controls usedin the dancer system. The inventor has found that the active variableforce component should generally be computed, and applied to the dancerroll, at a frequency that applies at least 5, preferably at least about100 control responses and more preferably at least about 200 controlresponses for each tension disturbance. Thus, if a given tensiondisturbance has a period of 10 seconds, then control responsesT*_(dancer) should be applied at least every 0.05 seconds.

Since, as discussed above, the first step in the control cycle issensing/measuring the several variables used in computing the variableforce component of the response, it is critical that the sensors measurethe variables frequently enough, to detect any tension disturbance thatshould be controlled early enough, to respond to and suppress thetension disturbance.

In order to have proper control of the dancer system 20, it is importantthat the computed responses be applied to the dancer roll frequentlyenough to control the dancer system. Again, at least 5 responses duringthe period of any tension disturbance is preferred. In order to providesufficient frequency in the response application, especially where thereis a variation in the frequency of occurrence of tension disturbances,it is preferred to measure the variables at a multiple of theanticipated desired frequency of applying a response.

Overall, the most critical frequency is the frequency of measuring thevariables shown as step 1 in the Flow Diagram, FIG. 3. Similarly, eachstep in the process must be repeated with a frequency at least as greatas the preferred frequency for applying the up-dated torque responsecommands.

The short-term tension disturbances addressed herein are typically lessthan 10 seconds in duration. Even shorter term tension disturbances,such as 0.67 second, 0.33 second, or even 0.2 second are readilycontrolled by the system disclosed herein. For example, a constantlyrepeated tension disturbance having a period of 10 seconds has afrequency of 6 cycles per minute. A period of duration of 0.67 secondsuggests a frequency of 100 cycles per minute. A period of duration of0.33 second suggests a frequency of 200 cycles per minute. A period ofduration of 0.2 second suggests 300 cycles per minute. Whatever thefrequency of the relevant tension disturbances to be controlled, oneneed only multiply the frequency of occurrence of the tensiondisturbances by a factor of 200 to arrive at a first estimation of anacceptable frequency of the response. A few trials with the operatingsystem, using modest variations of the frequency factor will reveal adesirable frequency for the particular processing system beingcontrolled by the dancer roll 24.

Thus, tension disturbances occurring at a frequency of 100 disturbancesper minute suggest a sensing frequency of at least 333 cycles persecond. Correspondingly, tension disturbance frequency of 200disturbances per minute suggest a sensing frequency, and correspondingresponse frequency of 667 cycles per second. Where a process is, forexample, cutting 300 items from the web per minute, or otherwiseimposing shocks on the web 300 times per minute, the sensors should besensing the variables, and the servo motor 56 should be applying arecomputed variable response force component, at least 1000 times persecond.

The dancer system 20 of this invention can advantageously be used withany dancer roll, at any location in the processing line. If there are noshort term tension disturbances in the web, the dancer roll will operatelike a conventional dancer roll. Then, when short term tensiondisturbances occur, the control system will automatically respond, toattenuate the short term tension disturbances.

Referring to FIG. 5, the dashed outline, represents calculations thatoccur inside the computer 70, with the resultant output of F*_(servo)being the output to the servo motor. The circle to the right of thecomputer controller represents the dancer roll 24, along with theseveral forces which act on the dancer roll. "M" represents the mass ofthe dancer roll 24; "g" represents gravity; and "P" represents theposition of dancer roll 24.

As used herein, the term "tension disturbance" means a sudden pull suchas to form a tuck, or a sudden relaxation as to temporarily eliminateall, or almost all, of the tension in the web. It includes all tensiondisturbances that can be significantly and finally attenuated by activeresponse of the dancer control system. Correspondingly, it excludesnormal increases and decreases in overall drive-line speed, which willoverwhelm the dancer system if not corrected at, for example, the unwindstation drive shaft. "Existence for no more than 10 seconds," referringto a tension disturbance includes disturbances that would last for morethan 10 seconds if not treated with the active dancer system, butexcludes disturbances where the active dancer treatment as disclosedherein cannot attenuate the entire disturbance within 10 seconds. Thus,the disturbances controlled by the control system of the invention caninclude single-step web take-ups such as disclosed in U.S. Pat. No.4,227,952 Sabee, as well as two-step disturbances wherein the tensionfirst is increased by a tension increase, and second is released over asimilar period of time, such as when e.g. a turning roll rotateseccentrically.

"Sensed tension" can refer to more than one sensing cycle, and more thanone location where the variable is sensed.

"Vertical velocity" means the translational velocity of the dancer roll24 within its operating window.

In the claims that follow, reference is made to a "first sensing andcontrol system" for sensing and controlling the static forces; and a"second sensing and control system" for sensing and controlling thedynamic forces. It should be understood that the first and secondsensing and control systems are not mutually exclusive. Rather, they usecommon sensors, and common controllers, thereby generating a combinedsingle output control force, based on the combination of forcecomponents attributable to the respective sensing and control systems.

The above described embodiments discuss the use of the dancer system 20with respect to attenuating tension disturbances in the web. Incorollary use, the dancer system 20 can also be used to create temporarytension disturbances. For example, in the process of incorporating lycrastrands or threads into a garment, e.g. at a nip between an underlyingweb and an overlying web, it can be advantageous to increase, ordecrease, the tension of the lycra at specific places as it is beingincorporated into each garment. The dancer control system 20 of theinvention can effect such short-term variations in the tension in thelycra.

Referring to FIG. 2, tension on the web is temporarily reduced oreliminated by inputting a force from servo motor 56 causing a sudden,temporary downward movement of the dancer roll, followed by acorresponding upward movement. Similarly, tension is temporarilyincreased by inputting a force from the servo motor 56 causing a sudden,temporary upward movement of the dancer roll, followed by acorresponding downward movement. Such a cycle of increasing anddecreasing the tension can be repeated more than 200 times, e.g. up to300 times per minute or more using the dancer system 20 of theinvention.

For example, to reduce the tension quickly and temporarily to zero, thecomputer controller commands, and the servo acts, to impose a temporarytranslational motion to the dancer roll during the short period overwhich the tension is to be reduced or eliminated. The distance of thesudden translational movement corresponds with the amount of tensionrelaxation, and the duration of the relaxation. At the appropriate time,the dancer is again positively raised by the servo to correspondinglyincrease the web tension. By such cyclic activity, the dancer roll canroutinely and intermittently impose alternating higher and lower (e.g.substantially zero). levels of tension on the web 18.

Having thus described the invention in full detail, it will be readilyapparent that various changes and modifications may be made withoutdeparting from the spirit of the invention. All such changes andmodifications are contemplated as being within the scope of the presentinvention, as defined by the following claims.

What is claimed is:
 1. Processing apparatus for advancing a continuousweb of material through a processing step wherein the web experiences anaverage dynamic tension along a given section of the web, the processingapparatus comprising:(a) a dancer roll operative for controlling tensionon the respective section of web; (b) actuating means (i) for applying afirst static force component to the dancer roll, through the firststatic force component having a first value and direction, and balancingthe dancer roll against the average dynamic tension in the respectivesection of the web, the dancer roll being passively responsive, throughthe first static force component, to thereby compensate for changes inweb tension lasting greater than about 10 seconds in duration, and (ii)for applying a second variable force component to the dancer roll, thecombination of the first static force component and the second variableforce component comprising a net actuating force; and (c) a computercontroller, connected to the actuating means, the computer controllerbeing adapted for controlling the net actuating force imparted to thedancer roll by the actuating means, and for adjusting the value anddirection of the second variable force component at least 1 time persecond, each such value and direction of the second variable forcecomponent replacing the previous such value and direction of the secondvariable force component, and acting in combination with the firststatic force component to impart a net translational velocity to thedancer roll, the second variable force component having a second valueand direction, modifying the first static force component, such that nettranslational velocity of the dancer roll is controlled by the netactuating force.
 2. Processing apparatus as in claim 1, said computercontroller being effective for adjusting the value and direction of thesecond variable force component at least 500 times per second. 3.Processing apparatus as in claim 1, said computer controller beingeffective for adjusting the value and direction of the second variableforce component at least 1000 times per second.
 4. Processing apparatusas in claim 1, including sensing means for sensing tension in the webafter said dancer roll, said computer controller being adapted to usethe sensed tension in computing the value and direction of the secondvariable force component, and for imparting the computed value anddirection to the actuating means.
 5. Processing apparatus as in claim 4,said sensing means being adapted for sensing tension at least 1 time persecond, said computer controller being connected to the actuating means,and being adapted for recomputing the value and direction of the secondvariable force component and thereby adjusting the value and directionof the computed second variable force component at least 1 time persecond, the actuating means being adapted to apply the recomputed secondvariable force component to the dancer roll at least 1 time per secondaccording to the values and directions computed by said computercontroller.
 6. Processing apparatus as in claim 4, said sensing meansbeing adapted for sensing tension at least 500 times per second, saidcomputer controller being connected to the actuating means, and beingadapted for recomputing the value and direction of the second variableforce component and thereby adjusting the value and direction of thecomputed second variable force component at least 500 times per second,the actuating means being adapted to apply the recomputed secondvariable force component to the dancer roll at least 500 times persecond according to the values and directions computed by said computercontroller.
 7. Processing apparatus as in claim 4, said sensing meansbeing adapted for sensing tension at least 1000 times per second, saidcomputer controller being connected to the actuating means, and beingadapted for recomputing the value and direction of the second variableforce component and thereby adjusting the value and direction of thecomputed second variable force component at least 1000 times per second,the actuating means being adapted to apply the recomputed secondvariable force component to the dancer roll at least 1000 times persecond according to the values and directions computed by said computercontroller.
 8. Processing apparatus as in claim 1, including sensingmeans for sensing tension in the respective section of the web withsufficient frequency to identify a tension disturbance in the web whichexists for a period of no less than about 10 seconds in duration, saidcomputer controller being adapted to recognize the period of existenceof recurring such tension disturbances and operative to adjust the valueand direction of the second variable force component at an adjustmentfrequency providing an average of at least about five adjustments duringthe existence of each such tension disturbance.
 9. Processing apparatusas in claim 1, including sensing means for sensing tension in therespective section of the web with sufficient frequency to identify atension disturbance in the web which exists for a period of less thanabout 0.67 second in duration, said computer controller being adapted torecognize the period of existence of recurring such tension disturbancesand operative to adjust the value and direction of the second variableforce component at an adjustment frequency providing an average of atleast about five adjustments during the existence of each such tensiondisturbance.
 10. Processing apparatus as in claim 1, including sensingmeans for sensing tension in the respective section of the web withsufficient frequency to identify a tension disturbance in the web whichexists for a period of less than about 0.33 second in duration, saidcomputer controller being adapted to recognize the period of existenceof recurring such tension disturbances and operative to adjust the valueand direction of the second variable force component at an adjustmentfrequency providing an average of at least about five adjustments duringthe existence of each such tension disturbance.
 11. Processing apparatusas in claim 1, including sensing means for sensing tension in therespective section of the web with sufficient frequency to identify atension disturbance in the web which exists for a period of less thanabout 0.2 second in duration, said computer controller being adapted torecognize the period of existence of recurring such tension disturbancesand operative to adjust the value and direction of the second variableforce component at an adjustment frequency providing an average of atleast about five adjustments during the existence of each such tensiondisturbance.
 12. Processing apparatus as in any one of claims 1-11therefore, said computer controller being adapted to compute the secondvariable force component using the equation

    T*.sub.dancer =r[F.sub.d static +b.sub.a (v*.sub.p -V.sub.p)+k.sub.a (F*.sub.c -F.sub.c)]

wherein r=radius of pulley on the servo motor, F_(d) static =staticforce component on the dancer roll and is equal to M_(g) +2F*_(c), M_(g)=mass of the dancer roll times gravity, F*_(c) =tension in the web,target set point, per process design parameters, b_(a) =control gainconstant re dancer translational velocity, in Newton seconds/meter,V*_(p) =translational velocity of dancer roll, set point, V_(p)=instantaneous translational velocity of the dancer roll immediatelyprior to application of the second variable force component, k_(a)=control gain constant re web tension, F_(b) =tension in the web aheadof the dancer roll, and F_(c) =tension in the web after the dancer roll.13. Processing apparatus for advancing a continuous web of materialthrough a processing step wherein the web experiences an average dynamictension along a given section of the web, the processing apparatuscomprising:(a) a dancer roll operative for controlling tension on therespective section of web; (b) a servo motor connected to said dancerroll and thereby providing an actuating force to said dancer roll; (c)first means for measuring a first velocity of the web after said dancerroll; (d) second means for measuring a second velocity of the web atsaid dancer roll; (e) third means for measuring translational velocityof said dancer roll; (f) fourth means for sensing the position of saiddancer roll; (g) fifth means for measuring web tension before saiddancer roll; (h) sixth means for measuring web tension after said dancerroll; and (i) a computer controller for providing control commands tosaid servo motor based on the sensed position of the dancer roll, andthe measured tensions and velocities, and thereby controlling theactuating force imparted to said dancer roll by said servo motor. 14.Processing apparatus as in claim 13, said computer controller beingadapted to provide control commands to said servo motor at a frequencyof at least 1000 times per second.
 15. Processing apparatus as in claim13, said computer controller being adapted to provide control commandsto said servo motor at a frequency of at least 1 time-per second. 16.Processing apparatus as in claim 13, said computer controller beingadapted to provide control commands to said servo motor at a frequencyof at least 500 times per second.
 17. Processing apparatus as in claim13, said computer controller being adapted to compute a control gainforce command using the equation

    T*.sub.dancer =r[F.sub.d static +b.sub.a (V*.sub.p -V.sub.p)+k.sub.a (F*.sub.c -F.sub.c)] wherein the dancer translational velocity set-point V*.sub.p is calculated using the equation

    V*.sub.p =EA.sub.o /(EA.sub.o -F.sub.c)[V.sub.2 (1-F.sub.b /EA.sub.o)-V.sub.3 (1-F.sub.c /EA.sub.o)],

and to control said servo motor based on the gain force so calculatedwherein F_(d) static =static force component on the dancer roll and isequal to M_(g) +2F*_(c), F_(c) =tension in the web after the dancer rollF*_(c) =tension in the web, target set point, per process designparameters, F_(b) =tension in the web ahead of the dancer roll, b_(a)=control gain constant re dancer translational velocity, in Newtonseconds/meter, k_(a) =control gain constant re web tension, M_(g) =massof the dancer roll times gravity, V_(p) =instantaneous translationalvelocity of the dancer roll immediately prior to application of thesecond variable force component, V₂ =velocity of the web at the dancerroll, V₃ =velocity of the web at the dancer roll, V*_(p) =translationalvelocity of dancer roll, set point, r=radius of pulley on the servomotor, E=Modulus of elasticity of the web, and A_(o) =cross-sectionalarea of the unstrained web.
 18. Processing apparatus as in claim 17,said computer controller being adapted to provide control commands tosaid servo motor at a frequency of at least 1 time per second. 19.Processing apparatus as in claim 17, said computer controller beingadapted to provide control commands to said servo motor at a frequencyof at least 500 times per second.
 20. Processing apparatus as in claim17, said computer controller being adapted to provide control commandsto said servo motor at a frequency of at least 1000 times per second.21. Processing apparatus for advancing a continuous web of materialthrough a processing step wherein the web experiences an average dynamictension along a given section of the web, the processing apparatuscomprising:(a) a dancer roll operative on the respective section of web;(b) means for applying a first static force component, having a firstvalue and direction, to the dancer roll, balancing the dancer rollagainst the average dynamic tension in the respective section of web,the dancer roll being passively responsive, through the first staticforce component, to thereby compensate for changes in web tensionlasting greater than about 10 seconds in duration; (c) a first sensingand control system for sensing the position of the dancer roll, andcontrolling the vertical position of the dancer roll by making operatingcondition adjustments over a period of time greater than 10 seconds, andthereby confining the dancer roll to an operating window; and (d) asecond sensing and control system for sensing, in the web, a shortduration tension disturbance existing for a period of less than about 10seconds in duration, and for imparting a corresponding counteractingforce component to the dancer roll, and thereby attenuating the effectof the short duration tension disturbance on the processing of the web.22. Processing apparatus for advancing a continuous web of materialserially through a processing step wherein the web experiences anaverage dynamic tension along a given section of the web, the processingapparatus comprising:(a) a dancer roll operative on the respectivesection of web; (b) means for applying a first static force component,having a first value and direction, to the dancer roll, balancing thedancer roll against the average dynamic tension in the respectivesection of web, the dancer roll being passively responsive, through thefirst static force component, to thereby compensate for changes in webtension lasting greater than about 10 seconds in duration; (c) a firstsensing and control system for sensing the position of the dancer roll,and controlling the vertical position of the dancer roll by makingoperating condition adjustments over a period of time greater than 10seconds, and thereby confining the dancer roll to an operating window;and (d) a second sensing and control system for creating a tensiondisturbance existing for less than about 10 seconds in duration in theweb by applying, to the dancer roll, a temporary and unbalancing forceof corresponding duration, thereby causing a temporary translationalmovement of the dancer roll and corresponding disturbance of the tensionin the web.
 23. Processing apparatus as in claim 22, said second sensingand control system being adapted for sensing a short duration tensiondisturbance in the web, existing for a period of less than about 0.67second in duration.
 24. Processing apparatus as in claim 22, said secondsensing and control system being adapted for sensing a short durationtension disturbance in the web, existing for a period of less than about0.33 second in duration.
 25. Processing apparatus as in claim 22, saidsecond sensing and control system being adapted for sensing a shortduration tension disturbance in the web, existing for a period of lessthan about 0.2 second in duration.